The present invention relates to a fired, porous alumina-based ceramic core impregnated with yttria to improve core creep resistance at elevated casting temperature used for casting metallic materials, especially reactive superalloys, and a method of improving core creep resistance by such impregnation.
In casting hollow gas turbine engine blades and vanes (airfoils) using conventional equiaxed techniques to produce equiaxed grain castings and directional solidification (DS) techniques to produce columnar grain or single crystal castings, a fired ceramic core is positioned in an investment shell mold to form internal cooling passageways in the airfoil. During service in the gas turbine engine, cooling air is directed through the passageways to maintain airfoil temperature within an acceptable range.
Green (unfired) ceramic cores typically are formed to desired core configuration by injection molding, transfer molding or pouring of an appropriate ceramic core material that includes one or more ceramic powders, a fugitive binder such as wax, polyproplylene, polyolefin, prehydrolized ethyl silicate, and other additives into a suitably shaped core die. After the green core is removed from the die, it is subjected to firing at elevated (superambient) temperature in one or more steps to remove the fugitive binder and sinter and strengthen the core for use in casting metallic material, such as a nickel or cobalt base superalloy. As a result of removal of the binder and fugitive filler material, if present, the fired ceramic core is porous.
The fired, porous ceramic cores used in investment casting of hollow turbine engine superalloy airfoils typically have an airfoil shape with a quite thin cross-section trailing edge region. U.S. Pat. No. 4,837,187 describes an alumina-based ceramic core formed, prior to sintering, from an admixture of alumina particles, yttria particles and a binder followed by debinding and sintering for use in investment casting of hollow airfoils from reactive superalloys, such as yttrium-bearing nickel base superalloys. Although such alumina-based cores have been used in production with success for years, the cores can exhibit a tendency to creep (move) at the high casting temperatures and extended time-at-temperature involved in DS casting of columnar grain and single crystal hollow superalloy airfoils (e.g. 1480 to 1600 degrees C. for xc2xd to 3 hours). As a result, positive core wall location and control features typically are provided in the form of platinum pins and/or alumina pins located between the core and ceramic shell mold wall in which the core is disposed at selected locations to maintain core position relative to the shell mold and counter the tendency of the core to creep at high DS (columnar grain and single crystal) casting temperature for extended time.
U.S. Pat. No. 5,580,837 describes an alumina-based ceramic core formed, prior to sintering, from an admixture of alumina particles, yttria aluminate particles and a binder followed by debinding and sintering for use in investment casting of hollow airfoils from reactive superalloys, such as yttrium-bearing nickel base superalloys.
An object of the present invention is to improve the creep resistance of such alumina-based cores as described in the above patents at elevated casting temperature used for casting metallic materials, such as especially reactive nickel based superalloys.
An embodiment of the present invention provides an impregnated fired porous alumina-based ceramic core for use in a mold in the casting of molten metals and alloys wherein the core is impregnated with yttria to improve core creep resistance at elevated casting temperature.
Another embodiment of the invention involves impregnating the fired porous alumina-based ceramic core with colloidal yttria or other impregnating medium that will provide yttria in pores of the core to improve core creep resistance.
Still a further embodiment of the invention provides an impregnated fired, porous alumina-based core that includes yttria impregnant in pores of the core and that exhibits a substantial increase in creep resistance at elevated casting temperature and time as compared to an unimpregnated fired, porous alumina-based core. The yttria impregnant in the core pores preferably is present in an amount of about 1% to about 5% by weight of the core (based on weight gain of the dried impregnated core).
The above objects and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings.